Model Atmosphere Selection

It is beyond the scope of this document to describe how to select an appropriate stellar atmosphere model for the analysis of a particular star, but the following comments should be of assistance. A stellar atmosphere model is characterized by 4 basic parameters, the effective temperature ($T_{\rm eff}$), the surface gravity ($\log g$), the overall metal abundance, [M/H], and the microturbulent velocity, $\xi_t$. These four parameters must also be specified to enable SPECTRUM to carry out its calculations. Of these four parameters, $T_{\rm eff}$ is certainly the most important, but the others cannot be ignored. There are many techniques that have been used in the literature to estimate the $T_{\rm eff}$. A carefully determined spectral type can yield the $T_{\rm eff}$ to an accuracy of $\pm 100$K, at least in the spectral-type range A - M. In addition, the process of spectral classification can reveal things about a star which you will need to know before carrying out a fine analysis - for instance, whether or not the star is chemically peculiar or deviates from the norm in any way. The $T_{\rm eff}$ may also be estimated from photometry; the infrared flux method (IRFM) is very useful here; Blackwell & Lynas-Gray (1994) have published a calibration that yields $T_{\rm eff}$ from the $V-K$ index. Other photometric calibrations can be found in the literature. D.F. Gray (Gray, 2008) has pointed out certain line ratios which may be used to estimate $T_{\rm eff}$. The surface gravity is more difficult to determine accurately; in the A and F-type stars the strength of the Balmer jump (measured by the c$_1$ index in Strömgren photometry, and less accurately by $U-B$ in Johnson photometry) is a sensitive measure of the gravity. When carrying out a fine analysis, an incorrect gravity will yield different abundances from different ionization states - for instance, if the iron abundance is different for Fe I and Fe II lines, the likelihood is that the gravity is wrong. Again, an accurate spectral type can be used to estimate the gravity from the luminosity type. For the A, F and G-type stars the paper by Gray, Graham & Hoyt (2001) may be useful in this regard. The determination of accurate abundances depends critically on the microturbulent velocity. The best way to proceed is to determine the abundance of a major species - say Fe I - simultaneously with the microturbulent velocity via a Blackwell diagram (see § ). Using a model atmosphere with a microturbulent velocity as close as possible to the value determined via the Blackwell diagram is important for precise work, as the microturbulent velocity affects the line strengths, hence the line opacity, which in turn affects the structure of the atmosphere itself.